How to measure the coastal currents of Akkaraipattu?

1. Where is Akkaraipattu?

Akkaraipattu is a town in the Ampara District of Sri Lanka. It is situated in the eastern part of the country, close to the Indian Ocean. The area is generally made up of a flat coastal plain that gradually gives way to the sea.

The landscape around the town is dominated by coconut palms and other tropical vegetation that grows well in the warm and humid climate. The local community has a strong attachment to the sea, with fishing being one of the major economic activities. The coastline around Akkaraipattu consists of sandy beaches and forms part of a larger coastal ecosystem supporting a wide variety of marine life.

The waters off Akkaraipattu are influenced by the general circulation patterns of the Indian Ocean. Relatively close to the ocean, the area is subject to tidal movements, and the exchange of water between the coastal zone and the open sea is an important factor in the local environment.

2. What is the situation of the coastal currents near Akkaraipattu?

Various factors shape the coastal currents near Akkaraipattu. Monsoon winds are the major factor. The southwest monsoon, from May to September, has strong winds that push the surface waters in a northeasterly direction. During the northeast monsoon, from December to February, the winds drive the currents in a southwesterly direction.

The topography of the seafloor near the coast affects the current patterns. The currents may accelerate, decelerate, or turn due to the presence of underwater sandbars, ridges, and channels. Another very important factor is that of tidal currents. This area receives semi - diurnal tides, or two high tides and two low tides in one day. The ebb and flow of the tides can lead to extreme changes in the current velocities and directions, interacting with the monsoon-driven and other oceanic currents to develop a complex flow regime.

It also depends on the exchange of water between the coastal area and the open ocean. Inflow and outflow of water into and from the region develop eddies and local current patterns that may differ from the overall ocean currents.

3. How to observe the coastal water flow of Akkaraipattu?

Surface Drifter Buoy Method

Surface drifter buoys offer a very straightforward and unproblematic way of observation of the surface water circulation. Such buoys are laid on the surface and moved by currents. The movement of buoys in such a way can be followed for some time by using satellite-based tracking systems or other methods of positioning. This allows one to learn the direction and speed of the surface currents. This, however, yields information concerning the surface layer alone and might not provide a detailed representation of the current structure throughout the whole water column.

Moored Ship Method

In this method, a ship is moored in a position from the coast. Instruments on board the ship record the flow of water at various depths. It enables the measurement continuously at one fixed point. However, it has its own drawbacks. The movement of the ship itself due to waves and wind may contribute to inaccuracies in the measurements. It also provides information on the area surrounding the moored ship and may not be representative of the general situation in the coastal currents.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP profiler method is a more advanced and comprehensive way to measure coastal currents. They produce acoustic pulses in the water and process the returning echoes which are Doppler -shifted. This allows the instruments to measure simultaneously the current velocity at more than one depth. The instruments are thus able to present an accurate profile of the current from the surface downwards, where the instrument capability allows, in detail. It gives more details on the present situation of the water column, hence it is an appropriate tool for the study on the coastal currents near Akkaraipattu.

4. How do Doppler principle-based ADCPs work?

ADCPs work by the principle of Doppler. They emit high-frequency acoustic signals into the water. As these signals meet the moving particles in the water, like suspended sediments or small organisms, due to the Doppler effect, the frequency of the reflected signals will shift.

If the particles are moving towards the ADCP meter, the frequency of the reflected signal is higher than the emitted frequency, and if they are moving away, it is lower. By measuring these frequency shifts at different angles and depths, the ADCP can calculate the velocity of the water at various points in the water column. Thus, it provides a current profile with details on the velocity and direction of the movement of water at various levels. From ADCPs, information is given that may help in understanding the spatial and temporal variabilities of coastal currents.

5. What's needed for high-quality measurement of Akkaraipattu coastal currents?

The equipment should have reliable materials for high-quality measurement of the coastal currents near Akkaraipattu. Since seawater is corrosive and the conditions are harsh in the sea, the equipment needs to be able to bear these conditions. The size should be compact, lightweight, and of low power consumption.

It also applies to small and light devices, which are much easier to deploy and retrieve, both from a boat and a coastal installation. Low power consumption allows longer-term monitoring, without the need to frequently replace batteries. Another crucial factor is cost-effectiveness, which is a decisive point for large-scale measurement projects.

The casing of the ADCP flow meter is preferably made of titanium alloy. Titanium alloy has excellent corrosion-resistant properties, making it well-suited for use in the marine environment. Besides, it has a good strength-to-weight ratio, which means it can provide the necessary structural integrity while keeping the device relatively light.

6. How to Choose the right equipment for current measurement?

Based on Usage

Ship - borne ADCP: If the measurements are to be taken from a moving vessel, a ship - borne ADCP is availed. It can provide continuous data as the ship travels along the coast and hence, is useful in understanding the spatial variability of the currents.

Bottom-mounted ADCP: A bottom-mounted ADCP current profiler is an excellent choice for long-term and fixed-point measurements near the seabed. The instrument can obtain detailed current profiles at one location, which will be very helpful in the study of local current behavior.

Buoy - mounted ADCP: When flexibility in deployment and measurement of surface and upper - water - column currents is required, a buoy - mounted ADCP is the way to go.

Based on Frequency

The choice of frequency is related to the water depth. For relatively shallow waters up to 70m, a 600kHz ADCP is usually appropriate. For water depths ranging between 70m to 110m, a 300kHz ADCP is a better fit. For even deeper waters, such as might be found near Akkaraipattu in some areas, a 75kHz ADCP will be suitable for up to approximately 1000m depth.

There are well - known ADCP current meter brands like Teledyne RDI, Nortek, and Sontek. However, for a cost - effective option with excellent quality, the China Sonar PandaADCP is a good alternative. It is made of titanium alloy and offers a great price - performance ratio. You can find more information at https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and moels.